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QUALITATIVE  EXAMINATION

presence of nitrogen. If the liquid has a blue colour, let it
stand for an hour and examine it again for a precipitate. If no
precipitate appears and the solution remains of a clear
yellowish-green colour, no nitrogen is present.
If sulphur is present, an excess of alkali metal must be
used to prevent the formation of sulphocyanide.
The Halogens.—Many halogen compounds impart a green
fringe to the outer zone of the non-luminous flame. A more
delicate test is to heat the substance with copper oxide
(Beilstein). Heat a fragment of copper oxide, held in the loop
of a platinum wire, in the outer mantle of the non-luminous
flame until it ceases to colour the flame green. Let it cool down
a little and then dust on some halogen compound (brom-
acetanilide will serve this purpose, see Prep. 55, p. 152). Now
heat again. A bright green flame, accompanied by a blue zone
immediately round the oxide, indicates the presence of a
halogen. The halogen in the majority of organic compounds
is not 'directly precipitated by silver nitrate. Only those
compounds which, like the hydracids and their metallic salts,
dissociate in solution into free ions give this reaction. If,
however, the organic compound is first destroyed, and 'the
halogen converted into a soluble metallic salt, the test may be
applied. JHeat the substance with a fragment of metallic sodium
or potassium as in the test for nitrogen, p. 2. The test-tube
whilst hot is placed in cold water, the alkaline solution filtered,
acidified with dilute nitric acid and silver nitrate solution added.
A curdy, white or yellow precipitate (provided no cyanide is
present), indicates a halogen. If a cyanide is present, boil with
nitric acid until the hydrogen cyanide is expelled and add
silver nitrate.
Sulphur.—The presence of sulphur in organic compounds
may be detected by heating the substance with a little metallic
sodium or potassium. The alkaline sulphide, when dissolved in
water, gives a violet colouration with a solution of sodium nitro-
prusside. Heat a fragment of gelatine with a small piece of
potassium in a test-tube until the bottom of the tube is 'red hot,
and place it in a small beaker of water as described in the test
for nitrogen (p. 2). Filter the liquid and add a few drops of
sodium nitroprusside solution.
Phosphorus.—The presence of phosphorus is ascertained